Causes of regular returns to estrus: the sow

Regular returns to estrus occur 18 - 24 days and 38 - 44 days post-service. A sow has a regular return (maintaining the normal duration of an estrous cycle: 18-24 days or its multiples) when fertilization of the eggs has failed or when all fertilized embryos have been lost within the first half of the estrous cycle (<10-12 days).

Under normal conditions these should make up a very small percentage of the matings, < 5%.

Sow-related causes

Management
1. Lactation length: Lactations shorter than 21 days predispose first parity sows to regular returns, and in multiparous sows the same is true for a lactation length less than 16 days.

Table 1. Sow-related factors that influence farrowing rate. Koketsu, Y. 1997

Lactation length (d)	n	Farrowing rate ± SE (sow level)	Farrowing rate ± SE (farm level)
8-10	146	79.7 ± 3.3cd	79.8 ± 5.1c
11-13	1005	80 ±1.2c	81.7 ± 3.1cd
14-16	1195	84.4 ± 0.6cd	83.6 ± 2.3cd
17-19	2749	83.9 ± 0.7cd	84.7 ± 2.3cd
20-22	2003	89.1 ± 0.7de	88.1± 2.7cd
23-28	682	91 ± 1.1e	90.2 ± 2.5dd

2. Recurring returns: Each time a sow returns and is serviced, she is more likely to return than when was serviced previously (the farrowing rate is estimated to be reduced by 10%). It is important to cull sows with more than two returns to estrus.

Table 2. Data from 106 comercial farms. Carlos Piñeiro. 2007

Total	1st service	1st return	2nd (or more) return
Number of matings	131,650	16,295	4,678
Total number of matings in the study	116,172	14,387	4,220
Returns	14,525	3,374	1,108
% Returns	12.5	23.5	26.3
Number of farrowings	91,153	8,713	1,780
Fertility (%)	84.4	68.1	52.2
Farrowing rate (%)	78.5	60.6	42.2
Adjusted farrowing rate (%)	82.4	66.4	52.7
Total born	12.1	12.0	11.6

3. Wean to service interval: Sows serviced late after weaning (7 - 12 days post-weaning) are usually less fertile.

Table 3. Fertility according to the wean to service interval. Koketsu, Y. 1997.

Wean to service interval (d)	n	Farrowing rate ± SE (sow level)	Farrowing rate ± SE (farm level)
1-6	6299	86.6 ± 0.4c	87.6 ± 1.9c
7-12	1333	81.8 ± 1.1d	76.1 ± 4.4d
≥ 13	900	85.4 ± 1.2c	85.4 ± 2.4c

4. Post-insemination management: Stress, in all its forms, can be one of the causes of sows having a regular return to estrus. Two times are especially critical:

• The first 30 minutes post-insemination. Stress during this time will block the oxytocin, responsible for sperm transport, which can affect the correct fertilization of the ova.
• The first 10-12 days post-service (may be heat stress). At this time stress can easily affect the correct implantation of the embryos leading to the loss of gestation.

Table 4. Influence of post-AI management. Flowers, B. 1996

	Frequency	Farrowing rate (%)	Born alive
Mixing sows	8//19	-7	-1.8
Moving sows	7/19	-10	-1.2

Overfeeding during the first 72 hours post-service can cause returns by reducing the levels of progesterone needed to maintain gestation.

Table 5. Effect of feeding level on embryonic survival. Aherne & Kirkwood. 1985

Feed rate		Embryonic survival		Progesterone
d1-3	d3-15	d28		ng/ml
1.9 kg	1.9 kg	12.4	86 %	10.5
2.5 kg	1.9 kg	11.5	77 %	3.7
2.6 kg	2.6 kg	10.2	67 %	4.5

Body condition

1. Sows that have lost more than 2 body condition points during lactation are more likely to return to estrus.

Table 6. Relationship between average intake during lactation and farrowing rate. Koketsu, Y. 1997.

Average feed intake, lactation (kg.)	n	Farrowing rate ± SE (sow level)	Farrowing rate ± SE (farm level)
≤ 4.2	2068	83.2 ± 0.8c	83.6 ± 2.5
4.2-5.7	4642	85.8 ± 0.5d	85.3 ± 2.4
≥ 5.7	2252	84.8 ± 0.8d	88.5 ± 2.3

2. Lameness, lesions: sows that are serviced being at a sub-optimal condition are more likely to return (Photo 1).

3. Toxicity or deficiencies: Lack of vitamins or amino acids, or presence of mycotoxins.

See the semen/boar-related causes for regular returns

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